Implantable cardiac devices have become increasingly sophisticated and more capable over time. The initial implantable cardiac devices were typically comprised of pacemakers, which provided electrical pacing pulses to the heart at a generally fixed rate. As the technology has developed, more advanced pacing systems have been implanted in patients, which, for example, are capable of providing pacing pulses to the heart only when the pacing system determines that the heart will not provide an intrinsic heart beat. Moreover, such advanced pacemakers are also able to adjust the pacing rate to accommodate different levels of physical activity and corresponding metabolic demand of the patient.
Typically, pacing systems are equipped with sensors, which provide signals that are used by the control unit of the pacing system to determine the pacing rate. Such sensors include activity sensors, such as an accelerometer, metabolic rate sensors, such as a minute ventilation sensor, electrical sensors, such as an impedance sensor, and pressure sensors.
Cardiac devices are also known to be able to perform automatic testing functions, such as threshold testing for automatic capture verification. Examples of known devices and methods include those described in U.S. Pat. Nos. 6,129,746; 6,243,606; 6,259,950; 6,263,244; 6,285,908; 6,295,471; 6,311,089; 6,366,812; 6,389,316; 6,408,210; 6,430,441; 6,477,417; 6,546,288; 6,584,354; 6,594,523; 6,618,622; 6,721,601; 6,766,197; 6,792,307; 6,925,326; and 6,934,587, each of which is incorporated by reference herein in its entirety.
Further, some cardiac devices are known to include lead impedance surveillance capabilities. For example, U.S. Pat. No. 7,031,773 to Levine et al., which is incorporated by reference herein in its entirety, describes a system in which an impedance measurement is triggered by a high capture threshold identified by an automatic capture algorithm.